lobj.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

    <para>
     To remove a large object from the database, call
<synopsis>
int lo_unlink(PGconn *conn, Oid lobjId);
</synopsis>
     <indexterm><primary>lo_unlink</></> The
     <parameter>lobjId</parameter> argument specifies the OID of the
     large object to remove.  Returns 1 if successful, -1 on failure.
    </para>
   </sect2>

</sect1>

<sect1 id="lo-funcs">
<title>Server-Side Functions</title>

  <para>
   There are server-side functions callable from SQL that correspond to
   each of the client-side functions described above; indeed, for the
   most part the client-side functions are simply interfaces to the
   equivalent server-side functions.  The ones that are actually useful
   to call via SQL commands are
   <function>lo_creat</function><indexterm><primary>lo_creat</></>,
   <function>lo_create</function><indexterm><primary>lo_create</></>,
   <function>lo_unlink</function><indexterm><primary>lo_unlink</></>,
   <function>lo_import</function><indexterm><primary>lo_import</></>, and
   <function>lo_export</function><indexterm><primary>lo_export</></>.
   Here are examples of their use:

<programlisting>
CREATE TABLE image (
    name            text,
    raster          oid
);

SELECT lo_creat(-1);       -- returns OID of new, empty large object

SELECT lo_create(43213);   -- attempts to create large object with OID 43213

SELECT lo_unlink(173454);  -- deletes large object with OID 173454

INSERT INTO image (name, raster)
    VALUES ('beautiful image', lo_import('/etc/motd'));

SELECT lo_export(image.raster, '/tmp/motd') FROM image
    WHERE name = 'beautiful image';
</programlisting>
  </para>

  <para>
    The server-side <function>lo_import</function> and
    <function>lo_export</function> functions behave considerably differently
    from their client-side analogs.  These two functions read and write files
    in the server's file system, using the permissions of the database's
    owning user.  Therefore, their use is restricted to superusers.  In
    contrast, the client-side import and export functions read and write files
    in the client's file system, using the permissions of the client program.
    The client-side functions can be used by any
    <productname>PostgreSQL</productname> user.
  </para>
</sect1>

<sect1 id="lo-examplesect">
<title>Example Program</title>

<para>
     <xref linkend="lo-example"> is a sample program which shows how the large object  
     interface
     in  <application>libpq</>  can  be used.  Parts of the program are 
     commented out but are left in the source for  the  reader's
     benefit.  This program can also be found in
     <filename>src/test/examples/testlo.c</filename> in the source distribution.
</para>

  <example id="lo-example">
   <title>Large Objects with <application>libpq</application> Example Program</title>
<programlisting>
/*--------------------------------------------------------------
 *
 * testlo.c--
 *    test using large objects with libpq
 *
 * Copyright (c) 1994, Regents of the University of California
 *
 *--------------------------------------------------------------
 */
#include &lt;stdio.h&gt;
#include &quot;libpq-fe.h&quot;
#include &quot;libpq/libpq-fs.h&quot;

#define BUFSIZE          1024

/*
 * importFile
 *    import file &quot;in_filename&quot; into database as large object &quot;lobjOid&quot;
 *
 */
Oid
importFile(PGconn *conn, char *filename)
{
    Oid         lobjId;
    int         lobj_fd;
    char        buf[BUFSIZE];
    int         nbytes,
                tmp;
    int         fd;

    /*
     * open the file to be read in
     */
    fd = open(filename, O_RDONLY, 0666);
    if (fd &lt; 0)
    {                           /* error */
        fprintf(stderr, &quot;can't open unix file %s\n&quot;, filename);
    }

    /*
     * create the large object
     */
    lobjId = lo_creat(conn, INV_READ | INV_WRITE);
    if (lobjId == 0)
        fprintf(stderr, &quot;can't create large object\n&quot;);

    lobj_fd = lo_open(conn, lobjId, INV_WRITE);

    /*
     * read in from the Unix file and write to the inversion file
     */
    while ((nbytes = read(fd, buf, BUFSIZE)) &gt; 0)
    {
        tmp = lo_write(conn, lobj_fd, buf, nbytes);
        if (tmp &lt; nbytes)
            fprintf(stderr, &quot;error while reading large object\n&quot;);
    }

    (void) close(fd);
    (void) lo_close(conn, lobj_fd);

    return lobjId;
}

void
pickout(PGconn *conn, Oid lobjId, int start, int len)
{
    int         lobj_fd;
    char       *buf;
    int         nbytes;
    int         nread;

    lobj_fd = lo_open(conn, lobjId, INV_READ);
    if (lobj_fd &lt; 0)
    {
        fprintf(stderr, &quot;can't open large object %d\n&quot;,
                lobjId);
    }

    lo_lseek(conn, lobj_fd, start, SEEK_SET);
    buf = malloc(len + 1);

    nread = 0;
    while (len - nread &gt; 0)
    {
        nbytes = lo_read(conn, lobj_fd, buf, len - nread);
        buf[nbytes] = ' ';
        fprintf(stderr, &quot;&gt;&gt;&gt; %s&quot;, buf);
        nread += nbytes;
    }
    free(buf);
    fprintf(stderr, &quot;\n&quot;);
    lo_close(conn, lobj_fd);
}

void
overwrite(PGconn *conn, Oid lobjId, int start, int len)
{
    int         lobj_fd;
    char       *buf;
    int         nbytes;
    int         nwritten;
    int         i;

    lobj_fd = lo_open(conn, lobjId, INV_WRITE);
    if (lobj_fd &lt; 0)
    {
        fprintf(stderr, &quot;can't open large object %d\n&quot;,
                lobjId);
    }

    lo_lseek(conn, lobj_fd, start, SEEK_SET);
    buf = malloc(len + 1);

    for (i = 0; i &lt; len; i++)
        buf[i] = 'X';
    buf[i] = ' ';

    nwritten = 0;
    while (len - nwritten &gt; 0)
    {
        nbytes = lo_write(conn, lobj_fd, buf + nwritten, len - nwritten);
        nwritten += nbytes;
    }
    free(buf);
    fprintf(stderr, &quot;\n&quot;);
    lo_close(conn, lobj_fd);
}

/*
 * exportFile
 *    export large object &quot;lobjOid&quot; to file &quot;out_filename&quot;
 *
 */
void
exportFile(PGconn *conn, Oid lobjId, char *filename)
{
    int         lobj_fd;
    char        buf[BUFSIZE];
    int         nbytes,
                tmp;
    int         fd;

    /*
     * open the large object
     */
    lobj_fd = lo_open(conn, lobjId, INV_READ);
    if (lobj_fd &lt; 0)
    {
        fprintf(stderr, &quot;can't open large object %d\n&quot;,
                lobjId);
    }

    /*
     * open the file to be written to
     */
    fd = open(filename, O_CREAT | O_WRONLY, 0666);
    if (fd &lt; 0)
    {                           /* error */
        fprintf(stderr, &quot;can't open unix file %s\n&quot;,
                filename);
    }

    /*
     * read in from the inversion file and write to the Unix file
     */
    while ((nbytes = lo_read(conn, lobj_fd, buf, BUFSIZE)) &gt; 0)
    {
        tmp = write(fd, buf, nbytes);
        if (tmp &lt; nbytes)
        {
            fprintf(stderr, &quot;error while writing %s\n&quot;,
                    filename);
        }
    }

    (void) lo_close(conn, lobj_fd);
    (void) close(fd);

    return;
}

void
exit_nicely(PGconn *conn)
{
    PQfinish(conn);
    exit(1);
}

int
main(int argc, char **argv)
{
    char       *in_filename,
               *out_filename;
    char       *database;
    Oid         lobjOid;
    PGconn     *conn;
    PGresult   *res;

    if (argc != 4)
    {
        fprintf(stderr, &quot;Usage: %s database_name in_filename out_filename\n&quot;,
                argv[0]);
        exit(1);
    }

    database = argv[1];
    in_filename = argv[2];
    out_filename = argv[3];

    /*
     * set up the connection
     */
    conn = PQsetdb(NULL, NULL, NULL, NULL, database);

    /* check to see that the backend connection was successfully made */
    if (PQstatus(conn) == CONNECTION_BAD)
    {
        fprintf(stderr, &quot;Connection to database '%s' failed.\n&quot;, database);
        fprintf(stderr, &quot;%s&quot;, PQerrorMessage(conn));
        exit_nicely(conn);
    }

    res = PQexec(conn, &quot;begin&quot;);
    PQclear(res);

    printf(&quot;importing file %s\n&quot;, in_filename);
/*  lobjOid = importFile(conn, in_filename); */
    lobjOid = lo_import(conn, in_filename);
/*
    printf(&quot;as large object %d.\n&quot;, lobjOid);

    printf(&quot;picking out bytes 1000-2000 of the large object\n&quot;);
    pickout(conn, lobjOid, 1000, 1000);

    printf(&quot;overwriting bytes 1000-2000 of the large object with X's\n&quot;);
    overwrite(conn, lobjOid, 1000, 1000);
*/

    printf(&quot;exporting large object to file %s\n&quot;, out_filename);
/*    exportFile(conn, lobjOid, out_filename); */
    lo_export(conn, lobjOid, out_filename);

    res = PQexec(conn, &quot;end&quot;);
    PQclear(res);
    PQfinish(conn);
    exit(0);
}
</programlisting>
</example>

</sect1>
</chapter>

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